Electric motors are the main means by which electrical energy is changed into mechanical energy. In industrial applications, electric motors range in size and power rating. There are several different types of motors in industrial use today. However, they can be grouped into two main categories, namely, brush-type and brushless. Brushless motors are in general made of a stator with a stator winding, and a rotor. The rotor can be made of only laminations, as is the case with the switched reluctance and synchronous reluctance motors. It can be made of a shaft with magnets mounted in different configurations, as in brushless motors, or permanent magnet synchronous motors can be used. Or, in the general case, the rotor can be combination of all the above technologies.
One thing all these technologies have in common is the need for an input voltage with variable magnitude and frequency to control them. Typically, a three-phase inverter is used for this task, and electronic commutation of a DC voltage is used to provide the variable voltage and frequency. In an ideal case, the use of a brushed DC motor would have eliminated the need to use a three-phase inverter in any application, especially ones that already have a provision for a variable DC voltage. In speed control applications, the use of a brushed DC motor would also eliminate the need for complex position sensing. In other cases, one would only need a variable DC voltage to control the motor, which would cut down the amount of electronics, and thus increase the efficiency and reduce the cost.
However, in downhole applications, the use of brushed DC motors is simply not possible because of the difficulty in placing the motor in air and applying a rotating seal that can withstand full differential pressure and motor torque. It is possible to magnetically couple the shaft torque of a brushed DC motor, but this is generally very inefficient. Placing the motor in oil will also not be possible because of the brushes and the commutator segments on the rotor need to be in contact in order to conduct electric current. The presence of an oil film between these two contacts will prevent proper conduction of current, and thus inhibit torque production.
The use of brushless motors, however, has some limitations. Particular difficulties in applying brushless motors downhole relate to the conventional use of electronic motor drives for communication and control of such motors. One of the main contributors to the development cost of a tool can be the development of such a motor drive. This is especially true in downhole tool development where the harsh environmental conditions limit the application of commercially available electronics. Thus, it is desirable—for at least some applications—to reduce or eliminate the requirement for power conversion electronics in brushless motor communication and control.
U.S. Pat. No. 6,239,531 to McGaughey and U.S. Pat. No. 6,667,564 to Tanh M. Bui et al both present mechanical commutator solutions having application in brushless motors. The '564 patent relates to an integrated motor/commutator system that relies on a particular timing cam and conducting terminals. The '531 patent relates to another integrated motor/commutator system that is characterized by a flexible conductive ring. Patent Publication No. WO 01/50578A1 to Pengov also describes a mechanical commutator, but one that is limited to driving a switched reluctance motor.
A need therefore exists for a mechanical commutator system for a brushless motor that is adaptive to downhole applications. For example, a need exists for such a commutator system that permits the physical separation of the commutator from the driven motor, e.g., using magnetic couplings, so as to permit the commutator and motor to be operated in discrete chambers or conditions.